Methods and apparatus of this type for distance measurement are generally known.
In the distance measurement, distance values should be obtained which are as precise as possible, on the one hand. On the other hand, a sensitivity of the measurement arrangement is aimed for which is as high as possible in order also to be able to detect signals with a comparatively low amplitude which originate, for example, from objects far away or from objects with a low reflectivity.
Known methods of distance measurement by determination of the pulse propagation time can be distinguished from one another with respect to the basic approach underlying them, with substantially two different approaches being used for the time measurement.
One approach is termed the “continuous measurement”. In continuous measurement, the total analog received signal is scanned, i.e. the received signal is measured “continuously” with a precision corresponding to the scan rate used. It is customary to carry out a plurality of measurements and to form an average value over the measurements carried out at a high fixed scan rate for every measuring position, i.e. at every scan position in the received signal, in order to improve the signal noise ratio also called S/N in this manner. The signal pulses actually of interest can be restored from the quasi-continuous amplitude resulting from this, i.e. they can be separated from the noise, in order to thus be able to carry out the time measurement required for the distance measurement with reference to the restored signal pulses.
The high precision which can be achieved with it and the high sensitivity are of advantage in the continuous measurement. The substantial technical problems which have to be solved with respect to the required effort for scanning, storing and computing are disadvantageous. The reason for these problems is the huge amount of data which has to be dealt with in the scanning of the total received signal and in the subsequent evaluation.
An alternative approach in the time measurement is the so-called “event measurement” in which it is not the whole analog received signal which is evaluated, but rather the received signal is led over a comparator which ensures that only signal portions or points in time exceeding a set comparator threshold are detected and evaluated. In contrast to the continuous measurement, a flood of data is consequently avoided from the start in the event measurement in that measured values are only recorded for the events actually of interest, in particular for the signal pulses lying above the comparator reference.
An advantage of the event measurement consists of a high precision being able to be achieved by averaging a plurality of measurements, while only a very small amount of data has to be coped with. It is a disadvantage that the event measurement naturally does not allow any improvement in the sensitivity since the sensitivity is fixed by the position of the comparator threshold which requires a specific spacing of the signal pulses of interest from the noise and which has to be set in accordance with this signal/noise spacing. A further disadvantage of the event measurement consists of the fact that only one single signal pulse, also known as an echo, can be evaluated per transmitted radiation pulse without an expensive and/or complex expansion of the evaluation electronics, since the transmitted radiation pulse starts the time measurement and the echo, i.e. the signal pulse detected at the receiver, stops the time measurement.
No measurement method is currently known in the field of distance measurement by pulse propagation time measurement which simultaneously delivers a high precision and a high sensitivity on the basis of a justifiable amount of data.